As of the date of this report, patients with idiopathic anaphylaxis (IA) continue to be admitted for study. The majority of the patients are admitted to the inpatient unit and undergo a history and physical examination, a blood draw for routine studies, and a bone marrow biopsy and aspirate in an attempt to elucidate the etiology and evaluate the pathogenesis of their disease. In collaboration with the NIH Clinical Center's myeloid core facility, we assess all the patient bone marrow aspirates and biopsies obtained based on the current WHO criteria to diagnose systemic mastocytosis. The randomized treatment protocol with omalizumab (09-I-0129) is in the patient accrual stage. Omalizumab is approved for the treatment of severe asthma and acts through a mechanism that down regulates the IgE receptor on the surface of basophils, mast cells, and dendritic cells. Omalizumab has been reported to be useful as adjunct therapy in the treatment of diseases other than asthma such as food allergy and chronic urticaria. Similarly, we have reported that omalizumab prevented episodes of spontaneous anaphylaxis in 2 patients with mastocytosis. There have been no serious adverse events contributed to the study drug, in particular drug-induced anaphylaxis.